The present invention relates to a radio control device for controlling an object at a remote place based on control data transmitted with radio waves. More particularly, the present invention relates to a multivalued FSK (Frequency Shift Keying) modulation system suitable for frequency-modulating carrier frequencies and transmitting and receiving multivalued data.
The radio-control (abbreviated as R/C) device for manipulating a moving device or instrument from the remote place by control information carried on radio waves is widely known. Model cars and model ships are an example of the moving objects to be operated by the radio wave.
Because such an apparatus generally uses narrow-band modulation signals permitted by Radio Wave Law, control is made using binary (or two-value) pulse data. However, when increasing the data transmission rate to improve the responsibility of the controlled device, the band width of the carrier frequency becomes wide. In consequence, it creates such a problem that intersymbol interference occurs between control signals in neighbor channels.
In order to overcome such a problem, it may be considered that control data is converted into a multi-valued value, e.g. two bits into one symbol to transmit the converted data so that the transmission rate can be increased without changing the time transmitting one symbol in a conventional system.
FIGS. 5(a) and (b) schematically shows a device for remotely transmitting two-valued data as four-valued data in accordance with the conventional system. In FIG. 5, numeral 11 represents a 2-to-4 value conversion circuit that receives digital input data of a binary (0, 1) for control. Numeral 12 represents a frequency modulator that frequency modulates a carrier frequency with data output converted by the 2-to-4 value conversion circuit. Numeral 13 is a high-frequency power amplifier that amplifies a modulated carrier signal and then feeds it to a transmission antenna 14.
A receiving antenna 21 receives radio waves transmitted from the transmitter 13. A high-frequency amplifier 22 amplifies the radio waves received by the antenna 21 to a signal of a constant level. The frequency detector 23 formed of a discriminator receives the amplified signal and then outputs it as a predetermined voltage level (e.g. a four-valued voltage).
In an embodiment of the present invention, four-valued dc voltage components are detected. However, the next 4-to-2 value conversion circuit 24 converts four-valued levels into a predetermined code signal.
In FIG. 5(c), the code signal is shown on the four-valued frequency/amplitude plane. For example, the frequency f0 corresponds to a 2-bit code signal of “00”. The frequency f1 corresponds to a 2-bit code signal of “01”. The frequency f2 corresponds to a 2-bit code signal of “11”. The frequency f3 corresponds to a 2-bit digital signal of “10”.
In the case of the four-valued FSK modulation system, when the same symbols of transmission data are sequentially produced, the frequency detector often produces outputs of the same amplitude level for a long period of time regardless of the 2-to-4 value conversion. Thus, it becomes difficult to perform the inter-symbol decoding of a digital signal (or to detect a sign point). As a result, the detecting data is erroneous.
In order to prevent such a problem, there are proposed modulation systems which insert a preamble signal during a predetermined period of time, while the same signs are not continuously output during encoding, or invert the next sign to the previous sign when the same data continues. However, these systems are disadvantageous in that the information transmission efficiency is markedly decreases, because extra data is added to the original data.